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I am currently doing experiments involving Software Defined Radio and Radar systems. Since I am new in this field and currently try to refresh my physics/math skills, I encountered some problems along the way.

The imaginary setup of my experiment is the following: A radar transmitter is placed at one location. An RF receiver (SDR) is stationed at another location. The radar is sending while the only job of the RF receiver is to receive the transmitted signals of the radar. At the same time, the radar is rotating and a difference in measured signal strength should be observed everytime the radar antenna is not facing the RF receiver. My main concern is to measure the moments in time, the radar antenna is directly facing the RF receiver.

However, I am rather confused on how to measure signal strength. Since I don't have access to a real radar system, I tried to implement one via Software-Defined Radio. The signal is generated by sending pulsed waves and to simulate a rotation of the antenna the amplitude increases for each pulse. There is one pulse with the peak amplitude in the middle. Afterwards the amplitude for each peak is declining. I am not sure if this is even remotely correct.

My question is whether it is possible to measure the time the radar antenna is directly facing towards the RF receiver and whether there are methods that can help me there.

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  • \$\begingroup\$ Consider using a detector chip like e.g. AD8319. It is a signal strength meter. \$\endgroup\$
    – Jens
    Jun 13, 2022 at 19:51
  • \$\begingroup\$ Also consider the radar antenna directionality; if it is outputting 90% of it's power in a 10 degree cone, then what you'll "see" is much less signal most of the time, and a very high peak when within that 10 degrees. \$\endgroup\$
    – rdtsc
    Jun 13, 2022 at 20:08
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    \$\begingroup\$ how accurate does the time have to be? \$\endgroup\$
    – jsotola
    Jun 13, 2022 at 22:21
  • \$\begingroup\$ when the received signal is the strongest, the antenna is no longer pointing directly at the receiver \$\endgroup\$
    – jsotola
    Jun 13, 2022 at 23:24
  • \$\begingroup\$ Why are you using a pulsed signal? Why not just continuously vary your simulated radar signal up and down, and write some software to detect to maximum. \$\endgroup\$
    – SteveSh
    Jun 13, 2022 at 23:45

2 Answers 2

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Finding the moment at which the signal strength reaches its peak is equivalent to finding local maxima of a mathematical function. If you continually calculate the first derivative of the signal strength with respect to time, then any time the derivative equals zero, that's probably going to be either a maximum or a minimum. In your application, there's probably only going to be one of each, so when the derivative is zero, that's probably going to be the maximum or the minimum.

To decide which, you probably can just look at whether the signal strength is greater than or less than some threshold. But, if you want to get fancy, and eliminate the need to choose a threshold, then you could also compute the second derivative. If the second derivative is negative when the first derivative is zero, then you've found the maximum.

See also: https://en.wikipedia.org/wiki/Fermat%27s_theorem_(stationary_points)

However, I am rather confused on how to measure signal strength.

Sorry, I can't help with that bit. I have not written code to interface with an SDR module.

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When you calculate a trigger level of e.g. 80% of the peak power you get two triggers per rotation around the flat high power angle. The center can be interpolatet and the precision improves, the more rotations are sampled. It would work like a PLL locks on an input signal. Spurios scatter is not a big problem with this method.

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